The present invention relates to scratch-resistant coatings for plastic articles, such as ophthalmic spectacle lenses and, in particular, to scratch-resistant coatings having improved adhesion to inorganic thin film coatings, such as antireflective coatings, mirror coatings and the like.
The application of scratch-resistant coatings to ophthalmic spectacle lenses is well known in the art. Scratch-resistant coatings are typically applied to plastic spectacle lenses, where the lens materials do not have a high degree of inherent scratch resistance—e.g., lenses made from injection molded polycarbonate or cast polymerizable monomers such as allyl diglycol carbonate. The scratch-resistant coatings are commonly applied by various techniques such as by spinning, dipping, or flowing the coating onto the substrate lens and curing the coating, usually by thermal or ultraviolet (UV) methods.
Another method of applying the scratch-resistant coating, referred to as “in-mold” coating, is described in U.S. Pat. Nos. 4,544,572 and 4,758,448, which are hereby incorporated by reference. This technique involves coating a mold used to form a cast lens with a polymerizable composition containing reactive ethylenically unsaturated groups, such as acrylates or vinyl compounds. The coating on the mold is then partially cured, such that some of the reactive groups remain available to form chemical bonds with the lens material. The coated mold is then used to cast an ophthalmic lens using a polymerizable composition also having reactive ethylenically unsaturated groups, such as allyl diglycol carbonate. When the lens material is cured and removed from the mold, the scratch-resistant coating is chemically bonded to the surface of the lens, imparting abrasion resistant properties.
Compositions used for in-mold scratch-resistant coatings frequently consist of multifunctional acrylic or methacrylic aliphatic monomers, multifunctional urethane(meth)acrylate oligomers or mixtures thereof. The term (meth)acrylate means the monomers or oligomers have either acrylate or methacrylate functionality. The resulting coating is hard and very abrasion resistant. The coating is optically clear, chemically resistant and resistant to degradation by sunlight (such as is evidenced by yellowing). Additionally, these coatings generally release relatively easily from the glass mold after polymerization of the lens. Such coatings also form chemical bonds with the lens materials during polymerization and, therefore, are very well bonded to the substrate lens.
In recent years, it has become popular to apply antireflective coatings to the surfaces of ophthalmic lenses. Antireflective coatings can significantly reduce the reflected light from the rear surface (the surface closest to the eye) of the spectacle lens, which is an annoyance to the wearer, and from the front surface of the lens, which improves the aesthetic appeal of the lens. Furthermore, because less light is reflected from the lens surfaces having antireflective coatings, more light is transmitted through the lens. It has also become popular to apply reflective inorganic thin film coatings to the surfaces of ophthalmic lenses particularly for certain lenses used for sunwear (eg. “mirror coatings”).
Antireflective coatings commonly consist of multiple thin layers of minerals, such as metal and metalloid oxides, having alternating relatively high and low index of refraction values. The ability of the first layer to adhere to the underlying scratch-resistant coating can affect how well the antireflective coating as a whole will adhere to the lens. Metal oxides and metalloid oxides such as silicon oxides often do not adhere well to organic scratch-resistant coatings—e.g., coatings made from multifunctional aliphatic(meth)acrylates and/or multifunctional urethane(meth)acrylate oligomers. Poor adhesion results in the gradual delamination of the antireflective coating over time and shortens the lifespan of the antireflective coating. Thus it would be desirable to provide a coating that is capable of being applied by in-mold coating techniques that is scratch-resistant and that has improved adhesion to antireflective coatings.